Non-aqueous electrolyte secondary cell cathode material and non-aqueous electrolyte secondary cell employing the cathode material

ABSTRACT

Carbon fiber having cross sectional shape which satisfies area replenishment rate of 0.8 or more is used as anode material for non-aqueous electrolyte secondary battery. Alternatively, since value of fractal dimension of cross section high order structure of the random radial type carbon fiber can be utilized as material parameter for evaluating the cross sectional structure, carbon fiber in which the value of the fractal dimension is caused to fall within the range from 1.1 to 1.8 and the crystallinity has been controlled such that it falls within reasonable range is used as anode material for non-aqueous electrolyte secondary battery. Further, carbon fiber having cross section high order structure such that the central portion is radial type structure and the surface layer portion is random radial type structure is used as anode material for non-aqueous electrolyte secondary battery. Furthermore, it is also effective to use carbon fiber having notch structure at the cross section. In addition, graphitized carbon fiber having cross sectional portions different in the crystal structure at predetermined periods in the fiber length direction is made up. By crushing the graphitized carbon fiber thus obtained, carbon fiber crushed powder having less unevenness and predetermined aspect ratio can be easily made up.

TECHNICAL FIELD

This invention relates to anode material for a non-aqueous electrolytesecondary battery, which consists of carbon material particularly carbonin fiber form, and further relates to a non-aqueous electrolytesecondary battery using such anode material.

BACKGROUND ART

Recent electronic technologies have conspicuously progressed so that,e.g., miniaturization and/or light weight of electronic equipments canbe realized in succession. Followed by this, also for batteries asportable power supply (supply) (source), there has been still moreincreased demand of miniaturization, light weight and high energydensity.

Hitherto, as the secondary battery of general use, aqueous solutionsystem batteries such as lead battery, or nickel/cadmium battery, etc.are the main current. These batteries can be satisfied to some extent inthe cycle characteristic, but it cannot be said that they havesatisfactory characteristic in points of battery weight and the energydensity.

On the other hand, studies/developments of non-aqueous electrolytesecondary batteries using lithium or lithium alloy as anode have beenextensively carried out in recent years. Such batteries have excellentcharacteristics of high energy density, small self-discharge and lightweight, but have the drawback that lithium is crystal-grown in dendriteform at the time of charging followed by development (progress) of thecharge/discharge cycle and reaches the cathode so that there results theinternal short. This is great hindrance to realization of practical use.

As a battery which solves such problem, non-aqueous electrolytesecondary batteries using carbon material as the anode, which are socalled lithium ion secondary battery, have been proposed and remarked.The lithium ion secondary battery utilizes doping/undoping of lithiuminto portion between carbon layers as the anode reaction. Even ifcharge/discharge cycle is developed, precipitation of crystal indendrite form cannot be observed at the time of charging. Thus, suchbatteries exhibit satisfactory charge/discharge cycle characteristic.

In this case, there are several carbon materials as carbon materialwhich can be used as the anode of the lithium ion secondary battery.Among them, material which has been first put into practical use is cokeand glass-shaped carbon. These materials are material having lowcrystallinity obtained after undergone heat-treatment at relatively lowtemperature, and has been commercialized as practical battery by usingelectrolytic solution mainly consisting of propylene carbon (PC).Further, also in graphite or the like which could not be used as anodewhen PC is used as main solvent, electrolytic solution mainly consistingof ethylene carbon (EC) is used so that arrival to usable level has beenrealized.

As the graphite or the like, graphite in a scale form can be relativelyeasily obtained. Hitherto, such graphite or the like has been widelyused as conductive material for alkali battery. This graphite or thelike advantageously high crystallinity and high true density as comparedto non-easily graphitized carbon material. Accordingly, if the anode isconstituted by the graphite or the like, high electrode filling (packingability can be obtained and the energy density of the battery is causedto be high. From this fact, it can be said that the graphite or the likeis greatly expected material as the anode material.

Meanwhile, most of the carbon materials exhibit form such as block form.In the case where such carbon materials are actually used as thebattery, they are crushed or pulverized and are used in powder form.

For this reason, even if the structure of carbon material is controlledso that the carbon material takes macro form or micro form by, e.g.,physical or chemical treatment (processing), there are actualcircumstances where the structure is disturbed by crushing, so itseffect cannot be sufficiently obtained.

On the contrary, in the case of carbon in fiber form (carbon fiber)obtained by carbonizing organic material in fiber form, it is easy torelatively control the carbon structure and there is no necessity ofcrushing. For this reason, such carbon fiber is advantageous whenapplication to the anode is assumed.

The structure of the carbon fiber greatly reflects the structure oforganic fiber which is precursor.

As organic fiber, there are organic fibers in which polymer such aspolyacrylonitrile, etc. is caused to be material, and organic fibers inwhich pitch or the like such as petroleum pitch, etc. and mesophasepitch caused to be oriented are caused to be material, etc. Theseorganic fibers all take fiber shape after undergone fiber-forming.

By carbonizing these organic fibers, carbon fibers can be obtained.However, since they are fused when heat-treated at the time ofcarbonization so that there results broken fiber structure, they arecarbonized after infusible processing is ordinarily implemented to thefiber surface by oxidation, etc.

The carbon fiber obtained in this way has cross sectional structureoriginating in the organic material fiber structure and exhibits highorder structure of, such as, for example, the type oriented inconcentrical form which is so called onion-skin type, the radiallyoriented radial type and isotropically random type, etc. Graphite fibersobtained by graphitizing these carbon fibers have high true density andalso have high crystallinity.

However, also in the above-described carbon fibers, it cannot be saidthat there is no problem.

Since, e.g., most of carbon fibers have circular cross section nearlyequal to complete round, in the case where they are filled (packed) intothe electrode, the so-called dead space takes place. Under thecircumstances where there is increased requirement of high energydensity with development of electronic equipments, the above-mentioneddead space constitutes great problem.

Moreover, in lithium ion secondary battery, since the intercalationreaction is the main anode reaction, it is known that according ascrystallinity of anode carbon material becomes higher, the capacitybecomes large. In the carbon fiber, with respect to the fiber crosssectional structure of the radial type, crystallinity is easy to beimproved, whereas crack is easy to take place in parallel to fiber axisby expansion/contraction at the time of charge/discharge and the fiberstructure is easy to be broken. Accordingly, in the carbon fiber of theradial type structure, large capacity can be obtained, but reversibilityof the charge/discharge cycle is not sufficient.

For this reason, as the anode carbon material, carbon fibers of therandom radial type in which the radial structure and the randomstructure are mixed are the main current. However, since the fiberdiameter is small and the cross section takes circular shape,rearrangement of the carbon layer surface is difficult to take place,thus making it possible to have high crystallinity, e.g., as in the caseof graphite in the scale form.

Further, in the case of the carbon fiber, since the orientation state ofthe cross section becomes uneven in the fiber length direction, there isalso the inconvenience that crack is apt to take place in the fiber axisdirection at the time of crushing and cutting. Since the carbon fiber isnot in block form as in the case of the ordinary graphite material,crushing is not required under strong condition. However, it isnecessary to finely crush and cut carbon fiber so that a fixed(predetermined) aspect ratio is provided. This crushing/cutting ofcarbon fiber involves various difficulties as compared to crushing ofcarbon material in block form. Thus, as previously described, not onlycrack is easy to take place, but also it is difficult to allow materialparameter such as aspect ratio, etc. to be fixed.

From these reason, there is nothing but to say that the battery made upby using conventional graphitized carbon fiber has insufficient capacityin the existing states and has low industrial reliability.

DISCLOSURE OF THE INVENTION

An object of this invention is to provide more practical carbon in fiberform (carbon fiber) which is high in the electrode filling (packing)ability, is excellent in crystallinity, is easy to be cut, and has lessvariations (unevenness) of material parameter, and to thereby provide anon-aqueous electrolyte secondary battery of high energy density andhigh reliability.

The inventors of this application have obtained various findings as theresult of the fact that they have energetically and repeatedly studied.This invention has been completed on the basis of these findings, andcontemplates to attain the above-described object by implementingvarious improvements to carbon fiber.

Namely, first of all, the cross sectional shape of the carbon fiber iscaused to be such a shape that area replenishment rate (degree) (valueobtained by dividing area of cross section of carbon fiber by product oflong side and short side of circumscribed rectangle which takes theminimum area in the case where the cross section of the carbon fiber isencompassed by the circumscribed rectangle) satisfies a specific range,0.8 or more in more practical sense.

Thus, anode material having high electrode filling (packing) ability andhaving less dead space is obtained.

Moreover, at this time, the cross sectional shape is caused to be such ashape to satisfy a specific range of circularity, whereby the cyclecharacteristic is further improved.

Secondly, since value of fractal dimension determined by the fractalanalysis of cross sectional high order structure of random radial typecarbon fiber can be utilized as material parameter for evaluating crosssectional structure, this value is caused to fall within a specificrange (from 1.1 to 1.8) to conduct a control such that crystallinity iscaused to fall within a reasonable range.

Thus, high capacity carbon fiber which is less in variations of thecharge/discharge ability and is satisfactory in the charge/dischargecycle reversibility can be realized.

Thirdly, in the high order structure of the carbon fiber, the centralportion is caused to be of radial type structure and the surface layerportion is caused to be of the random radial type structure.

Thus, carbon fiber having both strength tolerable toexpansion/contraction at the time of charge/discharge and high capacitycan be realized.

Fourthly, the cross sectional shape of the carbon fiber is caused to beof notch structure including notch (cut portion). The notch angle iscaused to be 2° to 150°.

Thus, even in the case where carbon fiber of the radial type structureis employed, carbon fiber having high capacity and strength tolerable toexpansion/contraction at the time of charge/discharge can be realized.

Fifthly, graphitized carbon fiber having cross sectional portions in thecrystal structure different from each other at predetermined period(interval) in the fiber length direction is made up. Then, thegraphitized carbon fiber thus obtained is crushed.

Thus, carbon fiber crushed powder having less unevenness and a fixedaspect ratio can be easily made up.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing cross section of carbon fiber in which thecentral portion is of radial type structure and the surface layerportion is of random radial type structure.

FIG. 2 is a view showing cross sectional shape of carbon fiber havingnotch structure.

FIG. 3 is a cross sectional view showing an example of shape ofdischarge hole for producing carbon fiber having notch structure.

FIG. 4 is a schematic perspective view showing graphitized carbon fiberhaving cross sectional portions different in the crystal structure atpredetermined period in the fiber length direction and carbon fibercrushed powder obtained by crushing the same.

FIG. 5 is a perspective view showing, in a model form, carbon fiberparticle model.

FIG. 6 is a cross sectional view showing an example of structure ofnon-aqueous electrolyte secondary battery.

FIG. 7 is a view showing an example of cross sectional shape ofgraphitized carbon fiber.

FIG. 8 is a view showing another example of cross sectional shape of thegraphitized carbon fiber.

FIG. 9 is a view showing a further example of cross sectional shape ofthe graphitized carbon fiber.

FIG. 10 is a view showing a more further example of cross sectionalshape of the graphitized carbon fiber.

FIG. 11 is a view showing a still more further example of crosssectional shape of the graphitized carbon fiber.

FIG. 12 is a view showing a further different example of cross sectionalshape of the graphitized carbon fiber.

FIG. 13 is a view showing a still further different example of crosssectional shape of graphitized carbon fiber.

FIG. 14 is a characteristic view showing the relationship between areareplenishment rate (degree) of fiber cross section and capacity.

FIG. 15 is a characteristic view showing the relationship betweencircularity of fiber cross section and capacity maintenance ratio.

FIG. 16 is a view showing an example of measurement of fractaldimension.

FIG. 17 is a characteristic diagram showing the relationship betweenspacing of (002) plane of carbon fiber and capacity.

FIG. 18 is a characteristic diagram showing the relationship betweenfractal dimension value and capacity maintenance ratio.

FIG. 19 is a model view showing an example of discharge hole used forproducing carbon fiber in which the central portion is of the radialtype structure and the surface layer portion is of the random radialtype structure.

FIG. 20 is a characteristic diagram showing the relationship betweenratio of the radial type structure and capacity.

FIG. 21 is a characteristic diagram showing the relationship betweenratio of the radial type structure and capacity maintenance ratio.

FIG. 22 is a characteristic diagram showing the relationship betweennotch angle in carbon fiber having notch structure and capacity.

FIG. 23 is a characteristic diagram showing the relationship betweennotch angle in carbon fiber having notch structure and capacitymaintenance ratio.

FIG. 24 is a characteristic diagram showing the relationship betweenaspect ratio of carbon fiber crushed powder and capacity maintenanceratio.

BEST MODE FOR CARRYING OUT THE INVENTION

In the anode material for non-aqueous electrolyte secondary battery ofthis invention, as carbon material for carrying out doping/undoping oflithium, there is used carbon fiber in which the area replenishment rate(degree) of the cross section is 0.8 or more, preferably 0.9 or more.

In this case, the area replenishment rate (degree) is defined as valueobtained by dividing area of cross section of fiber by product (area) oflong side and short side of circumscribed rectangle which takes theminimum area in the case where the cross section of fiber is encompassedby the circumscribed rectangle.

According as the area replenishment rate (degree) becomes closer to 1,the cross sectional shape of fiber becomes closer to rectangle, so thedead space taking place by curved surface is decreased. For this reason,the electrode filling (packing) ability can be enhanced. Accordingly,the energy density of the battery can be improved.

The area replenishment rate (degree) Y can be calculated by observingprofile (cross sectional) image of carbon fiber or its photograph bymicroscope such as electron microscope, etc. to determine area S ofprojected profile (cross sectional) image, and long side L and shortside B in the case where circumscribed rectangle which takes the minimumarea is taken with respect to the profile (cross sectional) image tocarry out substitution of these values into the following equation (1).

Area replenishment rate (degree) Y=S/(L×B)  (1)

In actual calculation, arbitrary 20 sample particles were extracted tocarry out similar calculation to allow its average value to berepresentative value of that material.

By further prescribing material within the above-mentioned range of thearea replenishment rate (degree) by circularity, the cyclecharacteristic can be improved.

The circularity is the value obtained by dividing circumferential lengthof circle having the same area as the area of projected profile image bylength of contour of the profile image. According as the profile imagebecomes close to circular shape, its value becomes equal to 1. In morepractical sense, the circularity can be determined by the followingmethod.

Namely, an approach is employed to observe profile image of carbon fiberor its photograph by microscope such as electron microscope, etc. todetermine circumference Lr of circle having the same area as the area Sof the protected profile image and contour length Lt of the projectedprofile image to carry out substitution of these values into theequation (2), thus to calculate circularity C.

Circularity C=Lr/Lt  (2)

In actual calculation, arbitrary 20 sample particles 20 were similarlyextracted to carry out similar calculation thereof to allow its averagevalue to be representative value of that material.

The value of circularity is preferably 0.8 or more and is less than 1.0,and is more preferably 0.9 or more and is less than 1.0. By allowing thevalue of circularity to fall within this range, the cycle characteristicis improved.

Although its reason is not certain, according as circularity becomeshigh, there results particle having lower flatness, so the bulk densityis increased and the electrode structure becomes satisfactory. For thisreason, it is estimated that the cycle life time is elongated.

In producing the carbon fiber, as organic material which is the startingmaterial, there can be used polymer or the like such aspolyacrylonitrile or rayon, petroleum system pitch, coal system pitch,synthetic pitch, and pitch or the like such as mesophase pitch, etc., inwhich these materials are held for an arbitrary time at about 400° C. atthe maximum, or they are caused to undergo polymerization promotion byaddition of acid, etc., whereby aromatic rings are condensed or arecaused to be polycyclic so that they are stacked and oriented.

Particularly, in the case where mesophase pitches are used, themesophase percentage content greatly affects fiber forming ability, andthe physical characteristic, or electric or chemical characteristic ofcarbon fiber. It is preferable that the mesophase percentage content is60% or more. It is further preferable that the mesophase percentagecontent is 95% or more. If the mesophase percentage is less than thisrange, orientation of crystal is inferior and lowering, etc. of capacityof material itself is caused. Therefore, this is not preferable.

In the case where organic fiber which is precursor of carbon fiber ismanufactured (produced), the above-mentioned polymer or the like or theabove-mentioned pitch or the like is heated so that it is in moltenstate. The material in molten state is formed as fiber after undergonemolding by discharge, etc. In this case, melting point is diverse independency upon respective organic materials. With respect to respectiveorganic materials, optimum fiber forming temperatures can be suitablyselected.

The structure of the carbon fiber greatly reflects the structure oforganic fiber which is precursor, particularly the cross sectional shapereflects the shape in carrying out fiber forming. For this reason, e.g.,in the case of extraction molding, it is important to select optimumshape of discharge hole.

Meanwhile, the structure of carbon fiber functioning as anode materialis classified into several structures in dependency upon its crosssectional structure. In more practical sense, there are structures ofcarbon fibers of onion-skin type oriented in a concentrical form, theradial type radially oriented and the isotropic random type, etc. Allcarbon fibers can be used as the anode material. Particularly, theradial type, or the random radial type in which the radial type and therandom type are mixed is suitable.

The value of fractal dimension of the graphitized carbon fiber crosssection having the random radial type structure will now be described.

The fractal dimension of the carbon fiber cross section is indexindicating the structure of carbon net plane in the fiber cross section.In order to obtain this fractal dimension, the electron microscope(electric field radiation type scanning electron microscope, etc.), etc.was first used to allow picture image of fiber cross section to bephotograph, etc. to take that picture image into the computer by usingscanner, etc. to allow it to undergo picture processing thereafter tocarry out fractal analysis. The average value determined with respect toten (10) fibers was caused to be value of fractal dimension (hereinaftersimply referred to as “FD value”).

The fractal dimension indicates degree of curvature of curve within theplane and takes value of 1 to 2. In this case, according as the curvebecomes complicated, value of the fractal dimension becomes close to 2.Namely, it is possible to quantitatively evaluate complicated carbonfiber cross sectional structure by the FD value. Particularly, the FDvalue is important as parameter for evaluating the curved structure ofthe carbon net plane which affects capacity as anode and reversibilityof charge/discharge cycle.

According as this value becomes close to 2, the curved structure becomescomplicated and the fiber strength becomes high. Thus, reversibility ofthe charge/discharge cycle is improved. However, on one hand, accordingas the curved structure becomes complicated, graphitization becomesdifficult. As a result, crystallinity is not improved and theintercurlation capacity is decreased.

For this reason, in order to allow crystallinity to be high and to allowreversibility of charge/discharge cycle to be satisfactory, it ispreferable that the FD value is 1.1 or more and is less than 1.8, and itis more preferable that the FD value is 1.25 or more and is less than1.8.

Moreover, as parameter of the crystal structure, (002) spacing d₀₀₂obtained by the X-ray analysis method (method of the Japan Society ofPromotion of Scientific Research) becomes index. In this case, it ispreferable that d₀₀₂ is less than 0.340 nm, it is more preferable thatd₀₀₂ is 0.335 nm or more and is 0.337 nm or less, and it is mostpreferable that d₀₀₂ is 0.335 nm or more and is 0.336 nm or less.

In order to control the crystallinity and the FD value, the startingmaterial and the method of conversion into fiber are important.

For example, in producing carbon fiber, as organic material which is thestarting material, there are used polymer or the like such aspolyacrylonitrile or rayon, etc., petroleum system pitch, coal systempitch, synthetic pitch, and pitch or the like such as mesophase pitch,etc. in which these materials are held for an arbitrary time at about400° C. at the maximum, or they are caused to undergo polymerizationpromotion by addition of acid, etc., whereby aromatic rings arecondensed or are caused to be polycyclic so that they are stacked andoriented.

Particularly, in the case where mesophase pitch is used, mesophasepercentage content greatly affects fiber forming and the physicalcharacteristic or electric or chemical characteristic of carbon fiber.It is preferable that the mesophase percentage content is 60% or more,and it is more preferable that the mesophase percentage content is 95%or more. If the mesophase percentage content is less than this range,orientation of crystal is inferior and lowering, etc. of capacity ofmaterial itself is caused. Therefore, this is not preferable.

In the case where organic carbon which is carbon fiber precursor ismanufactures (produced), the above-mentioned polymer or the like, orpitch or the like is heated so that it is in molten state. The moltenmaterial thus obtained is molded by discharge, etc. so that fiber isformed. In this case, melting point is diverse in dependency uponrespective organic materials, and optimum fiber forming temperatures canbe suitably selected with respect to respective organic materials.

Particularly, since the FD value is greatly affected by the fiberforming condition, i.e., extrusion speed in the case of the extrusionmolding and/or shape of discharge hole, etc., it is necessary toreasonably control these parameters. Moreover, in discharging, also byallowing flow of the pitch within the discharge hole to be turbulence,it is possible to form curved structure of carbon net plane in the crosssectional structure. In this case, there may be used a method in whichfine holes are provided at the discharge hole to blow off gas such asair, etc., a method in which magnetic field is caused to be produced inthe vicinity of discharge hole to disturb pitch orientation, and amethod in which discharge hole is vibrated by ultrasonic wave, etc.

The graphitized carbon fiber in which the central portion is of theradial type structure and the surface layer portion is of the randomradial structure will now be described.

In the high order structure of the carbon fiber cross section, itscentral portion is caused to be of the radial type structure and thesurface layer portion is caused to be of the random radial typestructure, whereby carbon fiber having both the strength tolerable toexpansion/contraction at the time of charge/discharge and high capacitycan be realized.

Namely, the cross sectional high order structure in this carbon fiber issuch that its central portion takes the radial type structure as shownin FIG. 1A. In the case of this radial type structure, orientation ofthe carbon layer surface is high. Particularly, high crystallinity iseasy to be obtained by high temperature heat treatment. On the otherhand, since fiber structure breakage by expansion/contraction at thetime of charge/discharge becomes apt to take place, the surface layerportion where crack takes place is caused to be of the random radialtype structure having high strength and relatively high crystallinity,whereby more practical carbon fiber having high intercurlation capacityand high strength tolerable to expansion/contraction at the time ofcharge/discharge is provided.

In the above-mentioned carbon fiber, if it includes many radial typestructures, the intercurlation capacity is increased, but fiberstructure breakage occurring by repetition of expansion/contraction atthe time of charge/discharge is apt to take place on the other hand. Forthis reason, the percentage content of the radial type structures shouldbe suitably selected according to use purpose of the buttery.

In the case where the cross section of the fiber carbon is circular,when the radius from its center is designated at R and the radius of theportion which forms the radial type structure in a concentrical form isdesignated at L as shown in FIG. 1B, the percentage content of theradial type structures can be prescribed by L/R.

Moreover, with respect to the cross sectional shape except for circularshape, when the geometric center of gravity is caused to be center andan arbitrary line is drawn from its center up to the cross sectionterminating portion, L/R is determined where its length is designated atR and the length of the portion which forms the radial type structure isdesignated at L. Further, lines are drawn at 15° intervals with the linebeing as reference to similarly determine L/R (values) with respect tothese respective lines to prescribe the average value thereof aspercentage content of the radial type structures.

It is preferable that the value of percentage content of the radial typestructures is 0.3 or more and is less than 1.0, it is more preferablethat the value of the percentage content is 0.5 or more and is less than1.0, and it is particularly preferable that the value of percentagecontent is 0.6 or more and is 0.9 or less.

It is to be noted that the radial type structure designates the portionradially oriented from the center of the fiber cross section, and theportion of this radial type structure can be confirmed by observing itordinarily by the scanning type electron microscope, etc. Moreover,since the radial type structure is the portion where an isotropy ofcrystal is high, it is possible to confirm it also by observing verysmall range by the polarization microscope or the transmission typeelectron microscope.

The carbon fiber takes at least two kinds of different structuresdivided in concentrical form in the case of circular shape in its crosssectional structure. In order to make up (manufacture) carbon fiberhaving such structure, it is necessary to control the structure ofpitch, etc. molten (fused) in the vicinity of the discharge hole outletat the time of manufacturing organic fiber.

As the above-mentioned control method, there may be applied any methodssuch as a method in which air, etc. is blown out within the dischargehole to change flow of orientation state of pitch, etc., a method inwhich magnetic field is applied from the external of the discharge holeto change flow of orientation state of pitch, etc., and a method inwhich the structure of the discharge hole itself is caused to bestructure divided into at least two sections or more in a concentricalform to change flow of pitch, etc. to thereby vary the orientationstate, etc.

The graphitized carbon fiber having notch structure will now bedescribed.

By allowing the high order structure of the carbon fiber cross sectionto be of notch structure, carbon fiber having high capacity and strengthtolerable to expansion/contraction at the time of charge/discharge canbe realized.

Namely, in the cross sectional high order structure in the carbon fiber,a structure such that a portion of carbon fiber shown in FIG. 2 islacking or missing, i.e., notch structure is provided in advance. Thus,even if the carbon portion is high crystallinity, the structuredistortion occurring by expansion/contraction taking place at the timeof charge/discharge can be absorbed by its notch portion. As a result,reversibility of the charge/discharge cycle can be improved.

In the above-mentioned notch structure, the cycle reversibility variesby difference of angle that the center and the fiber diameter outercircumference of the carbon fiber cross section shown in FIG. 2B form(hereinafter referred to as “notch angle”). Theoretically, it is knownthat the interlayer portion is swollen by about 10% by intercurlation tothe graphite structure. If the notch angle is small, the structuredistortion taking place by expansion/contraction occurring at the timeof charge/discharge fails to be absorbed. As a result, the structurebreakage takes place. On the other hand, if the notch angle is large,the carbon structure portion is decreased so that the capacity islowered. For this reason, while it is possible to suitably select thenotch angle in dependency upon the purpose, it is preferable that itsangle is 2° or more and is 150° or less.

With respect to the cross sectional shape except for circular shape,definition is made such that the geometrical center of gravity is causedto be center and angle that its center and the fiber outer circumferenceform is caused to be notch angle.

In measurement of the notch angle of the carbon fiber having notchstructure at the cross section, the fiber cross section is observed bythe electron microscope, thus making it possible to measure angle bypicture image or photographic picture image.

In the cross sectional high order structure in the carbon fiber,according as the ratio to take the radial type structure becomes high,higher capacity can be obtained. However, the above-mentioned structuredistortion has a tendency to take place. For this reason, there may besuitably selected the cross sectional structure such as the randomradial type, etc. in which, e.g., the random type structure except forthe radial type structure is mixed in dependency upon the purpose. Inthis case, since the strength of the carbon fiber itself is increased,the notch structure is provided, thereby permitting reversibility of thecharge/discharge cycle to be further satisfactory.

The above-mentioned carbon fiber having the notch structure can be madeup (prepared) by using discharge hole T having an outlet shape such thatbaffle plate is provided so that wedge is inserted in fiber-forming oforganic fiber as shown in FIG. 3, for example. However, the outlet shapeis not limited to such shape. In addition, if notch structure can beformed at the cross sectional structure of organic fiber which is theprecursor, any other methods can be applied.

A method of manufacturing carbon fiber crushed powder having lessunevenness of the material value parameter will now be described.

As shown in FIG. 4, graphitized carbon fiber serving as precursor(hereinafter simply referred to as “precursor graphitized fiber”) withrespect to carbon fiber crushed powder (hereinafter simply referred toas “crushed powder”) is caused to be of the structure having crosssectional portions different in the crystal structure at specific orpredetermined period (FIG. 4 shows the periodical structure of length 1)in the fiber length direction. This graphitized carbon fiber is crushedto form crushed powder serving as sample powder 22. Since the crystalorientation is different at different crystalline portions 21 differentin the crystal structure, the precursor graphitized fiber becomes easyto be broken by this portion at the time of crushing. Thus, samplepowder 22 of predetermined fiber length can be easily made up. In thiscase, the fiber diameter is caused to be d.

The structure of the carbon fiber greatly reflects the structure of theorganic fiber serving as the precursor. Accordingly, it is necessary toform the different crystalline portion 215 that the precursorgraphitized fiber 20 of this invention has while controlling the crystalorientation when the organic fiber is caused to undergo fiber-forming.

As the method of controlling the crystal orientation at the time pointof carrying out fiber-forming of organic fiber, there are method inwhich, in discharging, flow of pitch within the discharge hole is causedto be turbulence every certain length, a method in which fine holes areprovided at the discharge hole to blow out gas such as, etc., and amethod in which the discharge hole is vibrated by ultrasonic wave, etc.In addition, the property that pitch or the like serving as material isoriented with respect to magnetic field may be utilized.

Any methods of controlling crystal orientation except for the above maybe utilized. The important factor is the ratio that the differentcrystalline portions 21 are included within the precursor graphitizedfiber 20 or the interval (spacing) thereof.

Moreover, the different crystalline portions 21 may be caused to existin such a manner that they are distributed over the entirety of thecross section of the precursor graphitized fiber 20, or may be caused topartially exist. The existing state of the different crystallineportions 21 can be suitably selected in correspondence with materialparameter of required crushed powder. In this case, there are instanceswhere when the percentage content of the different crystalline portionsis increased, the intercalation capacity is decreased. It is preferablethat the percentage content is smaller.

With respect to orientation of the different crystalline portion 21,since it is necessary to break it in a direction perpendicular to thefiber axis by crushing, etc., it is preferable to orient the differentcrystalline portion 21 in the state nearly perpendicularly to the fibercross section. It is preferable that smaller angle that the differentcrystalline portion 21 forms with respect to the fiber axis is 60° ormore, and it is more preferable that such angle is 80° or more.

In the case where the spacing W between the different crystallineportions 21 existing in the precursor graphitized fiber 20 is short,material of small aspect ratio can be obtained. On the other hand, thereare instances where the percentage (content) of the differentcrystalline portions 21 becomes high so that the intercalation capacityis decreased. Further, in the case where this spacing W is long,material of large aspect ratio is provided. However, since thepercentage (content) of the different crystalline portions 21 becomeslower, loss of capacity is decreased. For this reason, while thisspacing can be suitably selected in correspondence with materialparameter or capacity of required crushed powder, it is preferable thatthe spacing is 1 d or more and is 100 d or less with respect to fiberdiameter d.

In making up (preparing) carbon fiber as previously explained, theorganic fiber which is the precursor of carbon fiber is caused to beinfusible state after fiber-forming and before heat treatment. Althoughits more practical means is not limited, there may be used, e.g., thewet method by aqueous solution such as nitric acid, mixed acid, sulfuricacid or hypochlorous acid, etc., the dry method by oxidizing gas (air,oxygen), and reaction by solid reagent such as sulfur, ammonium nitride,ammonium persulfate or ferric chloride, etc. In addition, in carryingout the above-mentioned treatment (processing), drawing or stretchingoperation may be carried out with respect to fiber.

The organic fiber which has been caused to undergo infusible treatment(processing) is heat-treated in inactive gas flow such as nitride, etc.In this case, as the condition, it is preferable to carbonize organicfiber at 300 to 700° C. thereafter to calcine it under the conditions oftemperature rising speed of 1 to 100° C. per min., arrival temperatureof 900 to 1500° C. and holding time of about 0 to 30 hours at thearrival temperature in the inactive gas flow to carry out heat treatmentat 2000° C. or more, preferably at 2500° C. or more in order to obtainfurther graphitized article. It is a matter of course that carbonizationor calcination operation may be omitted depending upon situations. Thecarbon fiber of this invention graphitized by carrying out heattreatment at high temperature of 2500° C. or more is preferable becauseit has true density close to artificial graphite and high electrodefilling (packing) density.

In this example, carbon fiber produced is applied for anode materialafter undergone milling or crushing/milling. At this time, crushing maybe carried out before and after carbonization or calcination, or in theprocess of temperature elevation before graphitization. In this case,heat treatment for graphitization is ultimately carried out in powderedstate.

In this invention, crushed powder of carbon fiber is used as anodematerial. In this case, materials of smaller aspect ratio exhibit highperformance. Accordingly, it is preferable that the aspect ratio ofcrushed powder is 50 or less and it is more preferable that the aspectratio is 10 or less. Moreover, it is preferable that the fiber diameterof the precursor graphitized fiber is more than 5 μm and is 100 μm orless, and it is more preferable that the fiber diameter is 8 μm or moreand is 60 μm or less. According as the fiber diameter becomes small, thespecific surface area becomes broader. In addition, according as thefiber diameter becomes greater, the effect for rendering fiber shape islowered to more degree. For this reason, this is not preferable.

In this example, the fiber diameter and the fiber length are determinedby observing crushed powder by using the electron microscope, etc.Moreover, value obtained by dividing its fiber length by the fiberdiameter is prescribed as the aspect ratio in that crushed powder. Thismeasurement is carried out with respect to ten (10) crushed powdersamples to allow respective average values to be fiber diameter, fiberlength and aspect ratio A.

Carbon fibers prescribed as described above respectively independentlyexhibit effects. In this case, by arbitrarily combining these effects,further great effects can be obtained.

For example, in the case where the carbon fiber has high order structureof the random radial type in the high order structure of the carbonfiber cross section, the value of fractal dimension and crystallinityare prescribed, whereby high capacity carbon fiber having lessunevenness in the charge/discharge performance and satisfactory in thecharge/discharge cycle reversibility can be provided. In this case, incarbon fibers having high order structure in which the central portionis of the radial type structure and the surface layer portion is of therandom radial type, value of the fractal dimension and/or crystallinityare prescribed at the portion of the random radial type, whereby thecharacteristic is further improved.

Moreover, the manufacturing methods in which the area replenishment rate(degree) and/or circularity of cross sectional shape of carbon fiber,and/or cross sectional portions different in the crystal structure atspecific or predetermined periods in the fiber length direction can beapplied also in carbon fibers of any cross sectional high orderstructure. By combination of these methods, anode material having highperformance at the industrial level can be obtained.

By allowing anode material to satisfy material values explained below,more practical anode material can be obtained.

In order to obtain higher electrode pack (filling) density, it ispreferable that the true density of graphitized carbon fiber is 2.1g/cm³ or more, and it is more preferable that the true density is 2.18g/cm³ or more. The true density of graphite material (pycnometer bybutanol solvent) is determined by its crystallinity, and crystalstructure parameters such as (002) spacing and C-axis crystallineelement thickness of (002) plane, etc. obtained by the X-ray diffractionmethod (method of the Japan Society for Promotion of the ScientificResearch) are caused to serve as index. In order to obtain material ofhigh true density, it is desirable to have higher crystallinity. It ispreferable that (002) spacing obtained by the X-ray diffraction methodis less than 0.340 nm, and it is more preferable that the (002) spacingis 0.335 nm or more and is 0.337 nm or less. In addition, with respectto the C-axis crystalline element thickness of (002) plane, it ispreferable that the thickness is 30.0 nm or more, and it is morepreferable that the thickness is 40.0 nm or more.

Moreover, in order to obtain satisfactory cycle characteristic, it ispreferable to use material having bulk density of 0.4 g/cm³ or more.Anode constituted by using graphite material having bulk density of 0.4g/cm³ or more has satisfactory electrode structure, and is difficult toexperience inconveniences such that graphite material slips off fromanode mix layer. Accordingly, long cycle life time can be obtained.

In this example, the bulk density prescribed here is value determined bythe method described in JIS K-1469. If graphite material having bulkdensity of 0.4 g/cm³ or more is used, sufficiently long cycle life timecan be obtained. It is preferable to use material having bulk density of0.8 g/cm³ or more. It is more preferable to use material having bulkdensity of 0.9 g/cm³ or more.

Bulk Density Measurement Method

The measurement method of bulk density will be indicated as below.

Measuring cylinder of volume of 100 cm³ of which mass has been measuredin advance is inclined to gradually throw sample powder of 100 cm³thereinto. Then, mass of the entirety is measured by the minimum scale0.1 g to subtract the mass of the measuring cylinder from its mass tothereby determine mass of the sample powder.

Then, cork stopper is put on the measuring cylinder in which the samplepowder is thrown to fall the measuring cylinder in that state 50 timesfrom height of about 5 cm with respect to rubber plate. Since the samplepowder within the measuring cylinder is compressed as the result offalling, volume V of the compressed sample powder is read. Thus, bulkdensity (g/cm³) is calculated from the following equation (3).

D=W/V  (3)

where

D is bulk density (g/cm³),

W is mass (g) of sample powder within the measuring cylinder, and

V is volume (cm³) of sample powder within the measuring cylinder after50 falling times.

Further, in the case where the average value of shape parameters xindicated by the following equation (4) is 125 or less, the cyclecharacteristic further becomes satisfactory. Namely, the representativeshape of graphite material powder is flat columnar shape as shown inFIG. 5a, or parallelepiped shape as shown in FIG. 5b. When the thicknessof the thinnest portion of this graphite material powder is designatedat T and the length of the longest portion thereof is designated at L,and the length in the direction perpendicular to the long axiscorresponding to depth is designated at W, product of values obtained byrespectively dividing L and W by T is the above-mentioned shapeparameter x. This shape parameter x means that according as its valuebecomes smaller, height with respect to the bottom area becomes higherand degree of flatness becomes smaller.

x=(W/T)×(L/T)  (4)

where

x is shape parameter,

T is thickness of the thinnest portion of powder,

L is length in the long axis direction of powder, and

W is length in the direction perpendicular to long axis of powder.

The average shape parameter x_(ave) mentioned here refers to valuedetermined by actual measurement as described below. Initially, graphitesample powder is observed by using SEM (Scanning type ElectronMicroscope) to select ten (10) powder samples such that the length ofthe longest portion is ±30% of the average particle diameter. Then,shape parameters x are calculated from the equation (4) with respect torespective selected ten (10) powder samples to calculate the averagevalue. The average value thus calculated is the average shape parameterx_(ave).

If the average shape parameter x_(ave) of graphite powder is 125 orless, the above-mentioned effect can be obtained. It is preferable thatthe value of the average shape parameter x_(ave) is 2 or more and isless than 115, and it is more preferable that the value of the averageshape parameter x_(ave) is 2 or more and is 100 or less.

Moreover, in the case where material having specific surface area of 9m²/g or less is used, longer cycle life time can be obtained.

This is because it is considered that micro particles of sub micronattached to graphite particles affect lowering of bulk density. Sincethe specific surface area is increased in the case where micro particlesare attached, employment of graphite powder of small specific surfacearea even in the case of similar grain size results in less influence bymicro particle. Thus, high bulk density can be obtained. As a result,the cycle characteristic is improved.

It is to be noted that the specific surface area mentioned here isspecific surface area measured and determined by the BET method.Although if the specific surface area of graphite powder is 9 m²/g orless, the above-mentioned effect can be sufficiently obtained, its valueis preferably 7 m²/g or less and its value is more preferably 5 m²/g orless.

Moreover, in order to obtain high safety and reliability as practicalbuttery, it is desirable to use graphite powder in which, in the grainsize distribution determined by the laser diffraction method, theaccumulated 10% particle diameter is 3 μm or more, the accumulated 50%particle diameter is 10 μm or more, and the accumulated 90% particlediameter is 70 μm or less.

Graphite powder filled (packed) into the electrode can be filled(packed) more efficiently when the grain size distribution is caused tohave width. A distribution closer to the normal distribution ispreferable. It is to be noted that there are instances where the batterymay be heated in abnormal state such as excessive charging, etc., andthe heat temperature has a tendency to rise in the case where particleshaving small diameter are distributed to much degree. Such a grain sizedistribution is not preferable.

Moreover, since graphite interlayer helium ions are inserted in chargingthe battery, crystalline elements are swollen by about 10% to press thecathode and/or the separator within the battery, resulting in the statewhere initial failure such as internal short, etc. is apt to take placeat the time of initial charging. In the case where particles of largeparticle diameter are distributed to much degree, occurrence rate offailure has a tendency to increase. For this reason, this is notpreferable.

Accordingly, there is used graphite powder having grain sizedistribution where particles of large particle diameter to particles ofsmall particle diameter are blended in a well balanced manner. Thus,practical battery having high reliability can be provided. In the casewhere shape of the grain size distribution is closer to the normaldistribution, particles can be filled more efficiently. In this case, itis desirable to use graphite powder in which, in the grain sizedistribution determined by the laser diffraction method, the accumulated10% particle diameter is 3 μm or more, the accumulated 50% particlediameter is 10 μm, and the accumulated 90% particle diameter is 70 μm orless. Particularly, in the case where the accumulated 90% particlediameter is 60 μm or less, initial failure can be reduced to muchdegree.

Further, in order to improve the heavy load characteristic as thepractical battery, it is desirable that average value of the breakagestrength values of graphite particles is 6.0 kgf/mm² or more.

Easiness of movement of ions at the time of discharge affects the loadcharacteristic. Particularly, in the case where a large number of holes(vacancies) exist in the electrode, since a sufficient quantity ofelectrolytic solution exists, satisfactory characteristic is exhibited.On the other hand, in graphite material having crystallinity, graphitehexagonal net planes are developed in the a-axis direction, and crystalof the c-axis is constituted by stacking thereof. Since coupling betweencarbon hexagonal net planes is coupling called van der Waals force, theyare apt to be deformed with respect to stress. For this reason, incompression-molding particles of graphite powder to fill them into theelectrode, they are easy to be collapsed or crushed as compared tocarbonaceous material baked at low temperation. Thus, it is difficult toensure holes (vacancies). Accordingly, according as the breakagestrength of graphite powder particles becomes higher, they are difficultto be collapsed and holes (vacancies) are easy to be produced. For thisreason, the load characteristic can be improved.

It is to be noted that the average value of breakage strength values ofgraphite particles mentioned here referrers to value determined byactual measurement as described below.

As measurement device for breakage strength, Shimazu Micro CompressionTester (MCTM-500) by Shimazu Seisaku Sho is used. Initially, graphitesample powder is observed by the optical microscope associated therewithto select ten (10) powder samples such that the length of the longestportion is ±10% of average particle diameter. Then, load is applied tothe selected respective ten (10) powder samples to measure breakagestrength of particles to calculate its average value. The average valuethus calculated is the average value of breakage strength values ofgraphite particles. In order to obtain satisfactory load characteristic,it is preferable that average value of breakage strength values ofgraphite particles is 6 kgf/mm² or more.

On the other hand, although cathode material used in combination withsuch anode consisting of carbon fiber or graphitized carbon fiber is notparticularly limited, it is preferable that the cathode materialincludes sufficient quantity of Li. For example, compound metal oxideconsisting of lithium and transition metal, or interlayer compoundincluding Li, etc. represented by the general expression LiMO₂ (Mindicates at least one of Co, Ni, Mn, Fe, Al, V, Ti) is suitable.

Particularly, since this invention aims at attaining high capacity, thecathode is required to include Li corresponding to charge/dischargecapacity of 250 mAh or more per anode carbon material 1 g in the steadystate (after, e.g., five charge/discharge operations are repeated). Itis more preferable to include Li corresponding to charge/dischargecapacity of 300 mAh or more.

It is to be noted that it is not necessarily required that Li ions areall delivered from the cathode material, but it is essentially requiredthat there exists Li (ion) corresponding to charge/discharge capacity of250 mAh or more per carbon material 1 g within the battery. In addition,quantity of Li (ions) is assumed to be judged by measuring dischargecapacity of the battery.

The anode material is used in the non-aqueous electrolyte secondarybattery. In this non-aqueous electrolyte solution secondary battery,non-aqueous electrolytic solution in which electrolyte is dissolved innon-aqueous solvent is used as electrolytic solution.

In this example, since graphite material is used as the anode in thisinvention, conventional propylene carbonate (PC) cannot be used as mainsolvent of non-aqueous solvent, and it is therefore the premise thatsolvents except for the above are used.

As solvent suitable as the main solvent, ethylene carbonate (EC) isfirst mentioned, but there is also suitable compound of the structure inwhich hydrogen element of EC is replaced by halogen element.

Moreover, although reactive with graphite material as in the case of PC,a portion of EC as main solvent or compound of the structure in whichhydrogen atom of EC is replaced by halogen element is replaced by a verysmall quantity of second component solvent, whereby satisfactorycharacteristic can be obtained. As the second component solvent, therecan be used PC, butylene carbonate, 1,2-dimethoxyethane,1,2-diethoxymethane, γ-butyrolactone, valerolactone, tetrahydrofuran,2-methyltetrahydrofuran, 1,3-dioxolan, 4-methyl-1,3-dioxolan, sulforan,methyl sulforan, etc. It is preferable that the second component solventis less than 10% by volume as its quantity of addition.

Further, the third component solvent may be added with respect to mainsolvent, or with respect to mixed solvent of the main solvent and thesecond component solvent to realize improvement in conductivity,decomposition suppression of EC and improvement in low temperaturecharacteristic, and to allow degree of reaction with lithium metal to below, thus to improve safety.

As the first mentioned solvent of the third component, chain carbonicester such as DEC (diethyl carbonate) or DMC (dimethyl carbonate), etc.is suitable. Moreover, unsymmetrical (asymmetrical) chain carbonic estersuch as MEC (methyl ethyl carbonate) or MPC (methyl propyl carbonate),etc. is suitable. The mixing ratio of chain carbonic ester which servesas the third component with respect to main solvent or mixed solvent ofmain solvent and the second component solvent (main solvent or mixedsolvent of main solvent and second component solvent: third componentsolvent) is preferably caused to fall within the range from 10:90 to60:40, and is more preferably caused to fall within the range from 15:85to 40:60.

As the solvent of the third component, mixed solvent of MEC and DMC maybe employed. It is preferable that the MEC-DMC mixing ratio is caused tofall within the range indicated by 1/9≦d/m≦8/2 when MEC volume isdesignated at m and DMC volume is designated at d. Moreover, it ispreferable that the mixing ratio between main solvent or mixed solventof the main solvent and the second component solvent and MEC-DMC whichserves as solvent of the third component is caused to fall within therange indicated by 3/10≦(m+d)/T≦7/10 when MEC volume is designated at m,DMC volume is designated at d and the entire volume of the solvent isdesignated at T.

On the other hand, as electrolyte dissolved into the non-aqueoussolvent, any one or more kinds of solvents which can be used in thebattery of this kind may be used in the mixed state as occasion demands.For example, LiPF₆ is preferable. In addition to the above, however,LiCIO₄, LiAsF₆, LiBF₄, LiB(C₆H₅)₄, CH₃SO₃Li, CF₃SO₃Li, LiN(CF₃SO₂)₂,LiC(CF₃SO₂) , LiCl, LiBr, etc.

More practical embodiments to which this invention is applied will nowbe described in detail on the basis of the experimental results.

Initially, studies have been made in connection with the areareplenishment rate (degree) and circularity of carbon fiber used asanode material.

Embodiment 1

(a) Manufacture of Anode Material

Coal system pitch was held for five (5) hours at 425° C. in theatmosphere of inactive gas to obtain coal system mesophase pitch ofsoftening point of 220° C. At this time, the mesophase percentagecontent was 92%.

The coal system mesophase pitch thus obtained was caused to undergodischarging and fiber-forming at a predetermined extraction pressure at300° C. to obtain precursor fiber. Thereafter, this precursor fiber wascaused to experience infusible processing at 260° C. to calcine theprecursor fiber thus processed at temperature of 1000° C. in theatmosphere of inactive gas to obtain carbon fiber. Further, this carbonfiber is heat-treated at temperature of 3000° C. in the atmosphere ofinactive gas to allow it to undergo air (wind) crushing to obtain samplepowder of graphitized carbon fiber.

The cross sectional shape of the sample powder thus obtained wasobserved by the electron microscopic observation to determine shape anddimensions of the fiber. Moreover, the area replenishment rate (degree)and the circularity were calculated. In this example, in calculation, itwas assumed that arbitrary twenty (20) sample particles are extracted touse its average value. The result is shown in Table 1.

The bulk density was determined by the method described in JIS K-1469.Similarly, its result is shown in the Table 1.

In addition, the cross sectional shape of the carbon fiber thus obtainedis shown in FIG. 7.

(b) Making Up of Cathode Active Material

Lithium carbonate of 0.5 mol and cobalt carbonate of 1 mol were mixed tobake this mixture for five (5) hours at temperature of 900° C. in air tothereby obtain LiCoO₂. As the result of the fact that the X-raydiffraction measurement was conducted with respect to the material thusobtained, its peak was well in correspondence with peak of LiCoO₂registered in JCPDS file.

Then, the sample powder was used as anode material to actually make upnon-aqueous electrolyte secondary battery of the cylindrical type. Theconfiguration of the battery is shown in FIG. 6.

(c) Making Up of the Anode 1

Graphitized carbon fiber powder 90 weight part and polyvinylidenefluoride (PVDF) 10 weight part as binding agent were mixed to preparedanode mix to disperse it into N-methyl pyrolidone serving as solvent toprepare anode mix slurry (in paste state) .

Belt-shaped copper foil of thickness of 10 μm was used as an anodecollector 10 to coat the anode mix slurry on both surfaces of thecollector to dry it thereafter to compression-mold it at predeterminedpressure to make up belt-shaped anode 1.

(d) Making Up of Cathode 2

The LiCoO₂ thus obtained was crushed so that LiCoO₂ powder such thataccumulated 50% particle diameter obtained by the laser diffractionmethod is 15 μm is provided. Then, this LiCoO₂ powder 95 weight part andlithium carbonate powder 5 weight part were mixed to mix 91 weight partof this mixture, graphite 6 weight part as conductive agent andpolyvinyledene fluoride 3 weight part as binding agent to preparecathode mix to disperse it into N-methyl pyrolidone to prepare cathodemix slurry (in paste state).

Belt-shaped aluminum foil of thickness of 20 μm was used as a cathodecollector 11 to uniformly coat the cathode mix slurry on both surfacesof this collector to dry it thereafter to compression-mold it, thus tomake up belt-shaped cathode 2.

(e) Assembling of the Battery

The belt-shaped anode 1 and the belt-shaped cathode 2 which have beenmade up in a manner as described above are stacked in order of the anode1, separator 3, the cathode 2, separator 3 through separator consistingof micro porous polypropylene film having thickness of 25 μm thereafterto wound it large number of times, thus to make up spiral type electrodebody having outside (outer) diameter of 18 mm.

The spiral type electrode body thus made up is accommodated within asteel battery can 5 to which nickel plating has been implemented. Then,insulating plates 4 are disposed at upper and lower both surfaces of thespiral type electrode to draw a cathode lead 13 of aluminum from anodecollector 11 so that it is caused to be conductive with a battery cover7 to draw an anode lead 12 of nickel from an anode collector 10 to weldit at the battery can 5.

Within this battery can 5, electrolytic solution in which LiPF₆ isdissolved at a ratio of 1 mol/l is poured into equi-volmetric mixturesolvent of EC and DMC. Then, the battery can 5 is caulked through asealing gasket 6 of which surface is coated by asphalt to fix a safetyvalve unit 8 having current interrupting mechanism, a PTC element 9 andthe battery cover 7 to keep air-tightness within the battery thus tomake up cylindrical non-aqueous electrolyte secondary battery havingdiameter of 18 mm and height of 65 mm.

Embodiment 2

Graphitized carbon fiber was manufactured similarly to the embodiment 1except that there is used precursor fiber obtained by holding coalsystem pitch for two (2) hours at 425° C. in the atmosphere of inactivegas to hold it for two (2) hours at 400° C. under methane gas flowthereafter to further hold it for twenty four (24) hours at 350° C. inthe atmosphere of inactive gas to allow the heat-treated coal systemmesophase pitch (mesophase percentage content is 95%) to undergodischarge and fiber-forming, and to further make up non-aqueouselectrolyte secondary battery.

Also in this example, the fiber shape and the average dimension aredetermined in a manner similar to the embodiment 1 with respect topowder of the graphitized carbon fiber thus obtained to calculate thearea replenishment rate (degree) and the circularity, and to measurebulk density.

The result is shown in the Table 1. In addition, the fiber crosssectional shape is shown in FIG. 8.

Embodiment 3

Graphitized carbon fiber was manufactured in a manner similar to theembodiment 1 except that there is used precursor fiber obtained byholding petroleum system pitch for three (3) hours at 430° C. in theatmosphere of inactive gas to allow heat-treated petroleum systemmesophase pitch having softening point of 210° C. to undergo dischargeand fiber-forming, and to make up non-aqueous electrolyte secondarybattery.

Also in this example, the fiber shape and the average dimension weredetermined in a manner similar to the embodiment 1 with respect topowder of the graphitized carbon fiber thus obtained to calculate thearea replenishment rate (degree) and circularity, and to measure bulkdensity.

The result is shown in the Table 1. In addition, the fiber crosssectional shape is shown in FIG. 9.

Embodiment 4

Precursor fiber was made up in a manner similar to the embodiment 3except that fiber-formable discharge hole which is more flat than thatof the embodiment 3 is used thereafter to manufacture graphitized carbonfiber in a manner similar to the embodiment 1, and to make upnon-aqueous electrolyte secondary battery.

Also in this example, the fiber shape and the average dimension weredetermined in a manner similar to the embodiment 1 with respect topowder of the graphitized carbon fiber thus obtained to calculate thearea replenishment rate (degree) and circularity, and to measure bulkdensity.

The result is shown in Table 1. In addition, the fiber cross sectionalshape is shown in FIG. 10.

Comparative Example 1

Precursor fiber was made up in a manner similar to the embodiment 1except that there is used fiber-formable discharge hole so that thefiber cross section takes right triangle thereafter to manufacturegraphitized carbon fiber in a manner similar to the embodiment 1, and tomake up non-aqueous electrolyte secondary battery.

Also in this example, the fiber shape and the average dimension weredetermined in a manner similar to the embodiment 1 with respect topowder of the graphitized carbon fiber thus obtained to calculate areareplenishment rate (degree) and circularity, and to measure bulkdensity.

The result is shown in the Table 1. In addition, the fiber crosssectional shape is shown in FIG. 11.

Comparative Example 2

Precursor fiber was made up in a manner similar to the embodiment exceptthat there is used fiber-formable discharge hole so that the fiber crosssection takes right triangle thereafter to manufacture graphitizedcarbon fiber in a manner similar to the embodiment 1, and to make upnon-aqueous electrolyte secondary battery.

Also in this example, the fiber shape and the average dimension weredetermined in a manner similar to the embodiment 1 with respect topowder of the graphitized carbon fiber thus obtained to calculate areareplenishment rate (degree) and circularity, and to measure bulkdensity.

The result is shown in the Table 1. In addition, the fiber crosssectional shape is shown in FIG. 12.

Comparative Example 3

Precursor fiber was made up in a manner similar to the embodiment 1except that there is used fiber-formable discharge hole so that thefiber cross section takes complete round shape thereafter to manufacturegraphitized carbon fiber in a manner similar to the embodiment 1, and tomake up non-aqueous electrolyte secondary battery.

Also in this example, the fiber shape and the average dimension weredetermined in a manner similar to the embodiment 1 with respect topowder of the graphitized carbon fiber thus obtained to calculate areareplenishment rate (degree) and circularity, and to measure bulkdensity.

The result is shown in the Table 1. In addition, the fiber crosssectional shape is shown in FIG. 13.

TABLE 1 CROSS SECTIONAL DIMENSION μm FIBER AREA BULK LONG DIAMETER ×LENGTH REPLENISHMENT DENSITY SHAPE SHORT DIAMETER μm RATE CIRCULARITYg/cm³ EMBODIMENT 1 SQUARE 30 × 30 70 1.00 0.89 0.92 EMBODIMENT 2RECTANGULAR 30 × 20 70 1.00 0.87 0.85 EMBODIMENT 3 ELLIPTIC 45 × 15 700.86 0.97 1.10 EMBODIMENT 4 ELLIPTIC 30 × 10 70 0.93 0.82 0.60COMPARATIVE RIGHT 30 × 15 70 0.50 0.70 0.50 EXAMPLE 1 TRIANGULARCOMPARATIVE EQUILATERAL 30 × 26 70 0.43 0.78 0.57 EXAMPLE 2 TRIANGULARCOMPARATIVE (COMPLETE    30 × 30 (φ30) 70 0.79 1.00 0.95 EXAMPLE 3ROUND) CIRCULAR

(Evaluation)

With respect to the respective batteries of the embodiments and thecomparative examples made up as described above, 2.5 hour constantcurrent/constant voltage charge operation was first carried out underthe condition of charge current of 1 A and the maximum charge voltage of4.2 V thereafter to carry out discharge operation under the condition ofdischarge current 700 mA until voltage falls down to 2.75 V to measurethe battery initial capacity. Its result is shown in Table 2. Inaddition, the relationship between the area replenishment rate (degree)and the battery initial capacity is shown in FIG. 14.

Then, the charge/discharge cycle was repeatedly carried out to determineratio of capacity (capacity maintenance ratio) of the 100-th cycle withrespect to capacity of the second cycle (capacity maintenance ratio). Inthe cycle test, charge operation was conducted for 2.5 hours under thecondition of the maximum charge voltage of 4.2 V and the charge currentof 1 A, and discharge operation was conducted under the condition ofconstant current of 700 mA until voltage falls down to 2.75 V. Thecapacity maintenance ratio was shown together in the Table 2. Inaddition, the relationship between circularity and capacity maintenanceratio is shown in FIG. 15.

TABLE 2 BATTERY CAPACITY CAPACITY MAINTENANCE RATIO mAh % EMBODIMENT 11589 92 EMBODIMENT 2 1572 90 EMBODIMENT 3 1463 95 EMBODIMENT 4 1526 81COMPARATIVE 1392 72 EXAMPLE 1 COMPARATIVE 1384 75 EXAMPLE 2 COMPARATIVE1415 89 EXAMPLE 3

As is clear from the Table 2 and FIG. 14, the carbon fibers of theembodiments 1 to 4 having the area replenishment rate (degree) whichfeatures this invention exhibited high electrode filling ability andhigh capacity.

Moreover, as is clear from the Table 2 and FIG. 15, the carbon fiberhaving a predetermined circularity which is the preferred embodimentalso exhibited high bulk density and high cycle capacity maintenanceratio.

It has been found that the non-aqueous electrolyte secondary batteryusing carbon fiber having a predetermined area replenishment rate(degree) and circularity has the excellent practical characteristic thathigh energy density and long cycle life time are caused to becompatible.

Studies were then conducted in connection with difference of thecharacteristic by fractal dimension value of the cross section.

Embodiment 5

Anode material was produced in a manner as described below.

Coal system pitch was held for five (5) hours at 425° C. in theatmosphere of inactive gas to obtain coal system mesophase pitch havingsoftening point of 220° C. At this time, mesophase percentage contentwas 90%. The coal system mesophase pitch thus obtained was extruded in apulse form, while applying ultrasonic wave thereto, by using dischargehole having inside diameter of 20 μm at 305° C. to allow it to undergodischarge and fiber-forming while changing pressure. Thus, precursorfiber was obtained. Thereafter, infusible processing was conducted at260° C. to calcine it at temperature of 1000° C. in the atmosphere ofinactive gas. Thus, carbon fiber was obtained. Further, the carbon fiberwas heat-treated at temperature of 3000° C. in the atmosphere ofinactive gas to allow the carbon fiber thus processed to undergo aircrushing/milling. Thus, sample powder of graphitized carbon fiber wasobtained.

As the result of the fact that (002) spacing d₀₀₂ and FD value of thesample powder thus obtained are determined, d₀₀₂ was equal to 0.3363 nm,and FD was equal to 1.1.

The method of measuring the fractal dimension (FD) value will now bedescribed.

Initially, in order to obtain picture image of the carbon fiber crosssectional structure, the cross section was observed under theacceleration voltage condition of 2 kv by using the electric fieldradiation type scanning electron microscope to take this still pictureinto the computer to divide the picture image thus obtained (FIG. 16A)into different five portions including the fiber central portions of 4μm×4 μm (consisting of pixels of 512×512) as shown in FIG. 16B. In thecase where luminance of picture image is not uniform, Fourie transformor filtering processing is implemented to carry out smoothing thereafterto binarize respective picture images. In the case where there isnecessity to more clarify the curve shape of the carbon network plane,image processing is implemented so that the white portion of thebinarized picture image can be recognized as thin line (FIG. 16C).

Calculation was carried out by using the equation (5) with respect tothe above-described five picture images to determine fractal dimensiond.

d=−Δ log N(1)/Δ log 1  (5)

where

1 is total number of squares when picture image is divided into squaresof certain size, and

N is the number of squares overlapping with curve of carbon net planewhen picture image is divided into squares of certain size.

Namely, picture image is divided into lattices consisting of squares tocalculate the number of squares overlapping with the curve of the carbonnet plane to similarly carry out calculation while changing the size ofsquare to determine average value with respect to five picture images.Further, such measurement was carried out with respect to ten (10)fibers to determine average value to allow it to be FD value.

The fractal dimension (FD) value can be measured also by the methoddescribed in, e.g., Carbon Material Society Bulletin “Carbon TANSO 1995,No. 169, P.207 to 214”.

The above-mentioned sample powder was used as the anode material to makeup cylindrical non-aqueous electrolyte secondary battery. The method ofmaking up (preparing) cathode active material, the method of making up(preparing) electrode and the method of assembling the battery aresimilar to those of the previously described embodiment 1.

Embodiment 6

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 5 except that discharge/fiber-formingwas carried out in a modified pulse condition to obtain precursor fiber.

The (002) spacing d₀₀₂ of the sample powder thus obtained was 0.3365 nm,and FD value was 1.2.

Embodiment 7

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 5 except that discharge/fiber-formingwas carried out in a further modified pulse condition to obtainprecursor fiber.

The (002) spacing d₀₀₂ of the sample powder thus obtained was 0.3367 nm,and FD value was 1.3.

Embodiment 8

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 5 except that discharge/fiber-formingwas carried out in a still further modified pulse condition to obtainprecursor fiber.

The (002) spacing d₀₀₂ of the sample powder thus obtained was 0.3372 nm,and FD value was 1.5.

Embodiment 9

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 5 except that discharge/fiber-formingwas carried out in a still more further modified pulse condition toobtain precursor fiber.

The (002) spacing d₀₀₂ of the sample powder thus obtained was 0.3363 nm,and FD value was 1.3.

Comparative Example 4

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 5 except that discharge/fiber-formingwas carried out in the state where no ultrasonic wave is applied to thedischarge hole to obtain precursor fiber.

The (002) spacing d₀₀₂ of the sample powder thus obtained was 0.3410 nm,and FD value was 1.8.

Comparative Example 5

Cylindrical non-aqueous solution secondary battery was made up in amanner similar to the embodiment 5 except that discharge/fiber-formingwas carried out in the state where no ultrasonic wave is applied to thedischarge hole to obtain precursor fiber.

The (002) spacing d₀₀₂ of the sample powder thus obtained was 0.3361 nm,and FD value was 1.0.

The result in the case where charge/discharge ability was measured withrespect to carbon fibers used in the respective embodiments andcomparative examples is shown in Table 3. In addition, the relationshipbetween the (002) spacing d₀₀₂ and capacity is shown in FIG. 17.

TABLE 3 FIBER CROSS SECTIONAL CAPACITY CROSS DIAMETER FIBER MAINTE-SECTIONAL DIMENSION LENGTH FD d002 CAPACITY NANCE STRUCTURE μm μm VALUEnm mAh/g RATIO % EMBODIMENT 5 RANDOM- 21 100 1.1 0.3363 300 81.7 RADIALEMBODIMENT 6 RANDOM- 22 100 1.2 0.3365 290 83.1 RADIAL EMBODIMENT 7RANDOM- 23 100 1.3 0.3367 275 85.3 RADIAL EMBODIMENT 8 RANDOM- 24 1001.5 0.3372 250 90.3 RADIAL EMBODIMENT 9 RANDOM- 26 100 1.3 0.3363 29285.6 RADIAL COMPARATIVE RANDOM- 20 100 1.8 0.3410 210 85.1 EXAMPLE 4RADIAL COMPARATIVE RADIAL 20 100 1.0 0.3361 330 68.3 EXAMPLE 5

The charge/discharge ability measurement method will now be described.Test cell described below was made up to carry out the measurement.

In preparing the test cell, pre-heat processing was first implemented tothe above-mentioned sample powder under the condition of temperaturerising speed of about 30° C./min., arrival temperature of 600° C. andarrival temperature holding time of one hour in the atmosphere of Ar.Thereafter, polyvinylidene fluoride corresponding to 10% by weight wasadded as binder to mix dimethyl formamide as solvent to dry it toprepare sample mix. Then, 37 mg of the sample mix was weighted to moldit along with Ni mesh serving as collector so that pellet havingdiameter of 15.5 mm is provided to make up working electrode.

The configuration of the test cell is as follows.

Cell shape: Coin type cell (diameter 20 mm, thickness 2.5 mm)

Opposite electrode: Li metal

Separator; polypropylene porous film

Electrolytic solution: Solution in which LiPF₆ is dissolved in mixedsolvent (1:1 in terms of volumetric ratio) of EC and DEC atconcentration of 1 mol/l.

The test cell constituted as described above was used to measurecapacity per 1 g of carbon material. In this case, doping of lithiuminto the working electrode (charging: strictly speaking, in this testmethod, charging is not carried out, but discharging is carried out inthe process where lithium is doped into carbon material. However, incorrespondence with the actual circumstances at actual battery, forconvenience, this doping process is called charging and undoping processis called discharging) was carried out under the condition of constantcurrent of 1 mA and constant voltage of OV (Li/Li+) per cell, and thedischarging (undoping process) was carried out under the condition ofconstant current of 1 mA per cell until terminal voltage falls down to1.5 V to calculate capacity at that time.

Then, with respect to the tubular (cylindrical) battery made up in therespective embodiments and comparative examples, there was repeatedlycarried out charge/discharge cycle in which 2.5 hour constant currentand constant voltage charge operation is conducted under the conditionof charge current 1 A and the maximum charge voltage of 4.2 V, anddischarge operation is then conducted under the condition of current 700mA until voltage falls down to 2.75 V. Thus, ratio of capacity of the 100-th cycle with respect to the capacity of the second cycle (capacitymaintenance ratio) was determined.

The result is shown in the above-mentioned Table 3. In addition, therelationship between FD value and capacity maintenance ratio is shown inFIG. 18.

From the above-described result, it has been found that the carbon fiberwhich has controlled the FD value serving as index of the crosssectional structure of this invention is anode material excellent in thecycle characteristic and the charging/discharging ability as compared tothe comparative example.

Difference of the characteristic resulting from difference of the highorder structure of the cross section of the carbon fiber was thenexamined.

Embodiment 10

In this example, anode material was made up in a manner as describedbelow.

Coal system pitch was held for five (5) hours at 425° C. in theatmosphere of inactive gas to obtain coal system mesophase pitch havingsoftening point of 220° C. At this time, the mesophase percentagecontent was 92%. The coal system mesophase pitch thus obtained wascaused to undergo fiber-forming by using a discharge tube 15 of thedouble structure composed of a discharge outer tube 15 a and a dischargeinner tube 15 b as shown in FIG. 19 at 300° C., thus to obtain precursorfiber.

In this example, the diameter A of the discharge outer tube 15 a was setto 20 μm, and the diameter B of the discharge inner tube 15 b was set to10 μm (B/A=0.5).

Thereafter, infusible treatment was conducted at 260° C. to calcine thearticle at temperature 1000° C. in the atmosphere of inactive gas toobtain carbon fiber. Further, heat treatment was conducted attemperature of 3000° C. in the atmosphere of inactive gas to carry outair crushing/milling to obtain sample powder of graphitized carbonfiber. The sample powder thus obtained has cross sectional shape by theelectron microscopic observation as shown in FIG. 1.

The above-mentioned sample powder was used as anode material to make upcylindrical non-aqueous electrolyte secondary battery. The method ofmaking up the cathode active material, the method of making up theelectrode and the method of assembling the battery are similar to thoseof the previously described embodiment 1.

Embodiment 11

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 10 except that the discharge hole ofB/A=0.7 is used to obtain precursor fiber.

Embodiment 12

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 10 except that discharge hole ofB/A=0.3 is used to obtain precursor fiber.

Embodiment 13

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 10 except that discharge hole ofB/A=0.1 is used to obtain precursor fiber.

Comparative Example 6

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 10 except that discharge hole of B/A=1is used to obtain precursor fiber having cross section of random radialstructure 100%.

Comparative Example 7

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 10 except that mesophase pitch ofmesophase percentage content 98% is used and discharge hole of B/A=1 isused to obtain precursor fiber having cross section of random radialstructure 100%.

Test cells similar to those in the case of the embodiments 5 to 9 weremade up with respect to carbon fibers used in the respective embodimentsand comparative examples to measure charge/discharge ability. Themeasurement results and the cross sectional shapes are shown in Table 4.The cross sectional shapes were observed by the electron microscope.

TABLE 4 CROSS SECTIONAL CHARGE/ CAPACITY DIAMETER FIBER DISCHARGEBATTERY MAINTE- DIMENSION LENGTH ABILITY CAPACITY NANCE SHAPE μm μm mAhg mAh RATIO % EMBODIMENT L/R = 0.45 SURFACE LAYER 20 100 315 1450 87 10PORTION IS RANDOM RADIAL, CENTRAL PORTION IS RADIAL EMBODIMENT L/R =0.68 SURFACE LAYER 20 100 323 1472 85 11 PORTION IS RANDOM RADIAL,CENTRAL PORTION IS RADIAL EMBODIMENT L/R = 0.31 SURFACE LAYER 20 100 3091436 85 12 PORTION IS RANDOM RADIAL, CENTRAL PORTION IS RADIALEMBODIMENT L/R = 0.11 SURFACE LAYER 20 100 303 1422 82 13 PORTION ISRANDOM RADIAL, CENTRAL PORTION IS RADIAL COMPARATIVE RANDOM RADIAL 20100 300 1410 84 EXAMPLE 6 COMPARATIVE RADIAL 20 100 330 1485 30 EXAMPLE7

Further, 2.5 hour constant current/constant voltage charge operation wascarried out under the condition of charge current of 1 A and the maximumcharge voltage of 4.2 V with respect to tubular (cylindrical) batteriesmade up in the respective embodiments and comparative examplesthereafter to carry out discharge operation under the condition ofdischarge current 700 mA until voltage falls down to 2.75 V to measurethe battery initial capacity. Its result is shown in the Table 4 andFIG. 20.

In addition, charge/discharge cycle is repeatedly carried out todetermine ratio of capacity of the 200-th cycle with respect to capacityof the second cycle (capacity maintenance ratio). In the cycle test,charge operation was carried out for 2.5 hours under the condition ofthe maximum voltage 4.2 V and the charge current 1 A, and dischargeoperation was carried out under the condition of discharge current of300 mA until voltage falls down to 2.75 V. The capacity of the secondcycle and the capacity maintenance ratio of the 200-th cycle withrespect to the second cycle are shown in the above-mentioned table 4 andFIG. 21.

From the above-described result, it has been found that the carbon fiberin which the central portion is of the radial type structure and thesurface layer portion is of the random radial structure can providebattery having well-balanced relationship between the battery capacityand cycle characteristic, high energy density, excellent cyclecharacteristic, and high reliability as compared to the comparativeexample.

The characteristic of the carbon fiber having notch structure was thenexamined.

Embodiment 14

Anode material was produced in a manner as described below.

Coal system pitch was held for five (5) hours at 425° C. in theatmosphere of inactive gas to obtain coal system mesophase pitch havingsoftening point 220° C. At this time, the mesophase percentage contentwas 92%. The coal system mesophase pitch thus obtained was caused toundergo fiber-forming by using discharge hole (angle that baffle plate Jforms is 3°) having diameter of 20 μm, which has shape shown in FIG. 3,at 300° C. to obtain precursor fiber. Thereafter, infusible processingwas conducted at 260° C. to calcine the article at temperature 1000° C.in the atmosphere of inactive gas to obtain carbon fiber. Notch angle ofthe fiber measured by the electron microscopic picture image was 2°.Further, heat treatment was conducted at temperature of 3000° C. in theatmosphere of inactive gas to carry out air crushing/milling. Thus,sample powder of graphitized carbon fiber was obtained.

The above-mentioned sample powder was used as the anode material to makeup cylindrical non-aqueous electrolyte secondary battery. The method ofmaking up cathode material, the method of making up electrode and themethod of assembling battery are similar to those of the previouslydescribed embodiment 1.

Embodiment 15

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge hole inwhich angle that baffle plate J forms is 10° is used to obtain precursorfiber (notch angle is 8°).

Embodiment 16

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge hole inwhich angle that baffle plate J forms is 30° C. is used to obtainprecursor fiber (notch angle is 28°).

Embodiment 17

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge hole inwhich angle that baffle plate J forms is 50° is used to obtain precursorfiber (notch angle is 47°)

Embodiment 18

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge hole inwhich angle that baffle plate J forms is 70° is used to obtain precursorfiber (notch angle is 72°).

Embodiment 19

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge hole inwhich angle that baffle plate J forms is 90° is used to obtain precursorfiber (notch angle is 88°).

Embodiment 20

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge hole inwhich angle that baffle plate J forms is 125° is used to obtainprecursor fiber (notch angle is 120°).

Comparative Example 8

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge holeincluding no baffle plate J is used to obtain precursor fiber (nonotch).

Comparative Example 9

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that pich in which themesophase percentage ratio is 30% is used and discharge hole includingno baffle plate J is used to obtain precursor fiber (no notch).

Comparative Example 10

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 14 except that the discharge hole inwhich angle that baffle plate J forms is 145° is used to obtainprecursor fiber (notch angle is 140°).

Test cells similar to those in the cases of the embodiments 5 to 9 weremade up with respect to the carbon fibers used in the respectiveembodiments and comparative examples to measure charge/dischargeability. Its measured result is shown in Table 5. In addition, therelationship between notch angle and capacity is shown in FIG. 22.

TABLE 5 CROSS FIBER CROSS SECTIONAL FIBER CAPACITY SECTIONAL DIAMETERLENGTH NOTCH CAPACITY MAINTENANCE STRUCTURE DIMENSION μm μm ANGLE °mAh/g RATIO % EMBODIMENT 14 RADIAL 20 100 2 327 80.3 EMBODIMENT 15RADIAL 20 100 8 321 85.2 EMBODIMENT 16 RADIAL 20 100 28 303 92.5EMBODIMENT 17 RADIAL 20 100 43 284 91.5 EMBODIMENT 18 RADIAL 20 100 72266 92.0 EMBODIMENT 19 RADIAL 20 100 88 248 91.3 EMBODIMENT 20 RADIAL 20100 120 223 88.2 COMPARATIVE RADIAL 20 100 NONE 331 72.1 EXAMPLE 8COMPARATIVE RANDOM 20 100 NONE 210 85.0 EXAMPLE 9 COMPARATIVE RADIAL 20100 140 209 80.0 EXAMPLE 10

Further, with respect to the tubular (cylindrical) batteries made up inthe respective embodiments and comparative examples, there wasrepeatedly carried out charge/discharge cycle in which 2.5 hour constantcurrent/constant voltage charge operation is then carried out under theconditions of the charge current of 1 A and the maximum charge voltageof 4.2 V and discharge operation is carried out under the condition ofdischarge current 700 mA until voltage falls down to 2.75 V. Thus, ratioof capacity of the 100-th cycle with respect to capacity of the secondcycle (capacity maintenance ratio) was determined. The result of thecapacity maintenance ratio of the 100-th cycle with respect to thesecond cycle is shown in the above-mentioned table 5. In addition, therelationship between notch angle and capacity maintenance ratio is shownin FIG. 23.

From the above-described result, it has been found that notch isprovided at carbon fiber so that anode material excellent in the cyclecharacteristic is provided.

The performance as the anode material of the carbon material formed bycrushing carbon fiber having cross sectional portions in which crystalstructures are different periodically in the fiber length direction.

Embodiment 21

Anode material was produced in a manner as described below.

Petroleum system pitch was held for five (5) hours at 425° C. in theatmosphere of inactive gas to obtain petroleum system mesophase pitchhaving softening point 230° C. At this time, the mesophase percentageratio was 91%. The petroleum system mesophase pitch thus obtained wascaused to undergo discharge and fiber-forming at a predeterminedextraction pressure at 300° C. while applying magnetic field in a pulseform every predetermined time interval by using discharge hole havinginner diameter of 20 μm within which small probe for application ofmagnetic field is included. Thus, organic fiber was obtained.Thereafter, this organic fiber was caused to experience infusibleprocessing at 260° C. to calcine it at temperature of 1000° C. in theatmosphere of inactive gas to obtain carbon fiber. Further, the carbonfiber thus obtained is heat-processed at temperature 3000° C. in theatmosphere of inactive gas to allow it to be precursor graphitized fiberas shown in FIG. 4a to further air-crush the precursor graphitized fiberto allow it to be sample powder as shown in FIG. 4b. The aspect ratio ofthe sample powder thus obtained was expressed as A=1.3 and the specificsurface area was 0.9 mm²/g.

The above-mentioned sample powder was used as anode material to make uptubular (cylindrical) non-aqueous electrolyte secondary battery. Themethod of making up the cathode active material, the method of making upelectrode and the method of assembling battery are similar to those ofthe previously described embodiment 1.

Embodiment 22

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 21 except that discharge/fiber-formingis carried out in the modified application pulse condition of magneticfield to obtain organic fiber. The aspect ratio of the sample powderthus obtained was expressed as A=3.3 and the specific surface areathereof was 0.8 m²/g.

Embodiment 23

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 21 except that discharge/fiber-formingis carried out in the further modified application pulse condition ofmagnetic field to obtain organic fiber. The aspect ratio of the samplepowder thus obtained was expressed as A=7.0 and the specific surfacearea thereof was 1.2 m²/g.

Embodiment 24

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 21 except that ultrasonic wave isapplied to the discharge hole front end in a pulse form in place ofmagnetic field to carry out discharge/fiber-forming to obtain organicfiber. The aspect ratio of the sample powder thus obtained was expressedas A=9.3 and the specific surface area was 1.3 m²/g.

Embodiment 25

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 21 except that discharge hole havingfine holes therewithin is used in place of magnetic field to carry outdischarge/fiber-forming while ejecting air in a pulse form from the fineholes to obtain organic fiber. The aspect ratio of the sample powderthus obtained was expressed as A=41.0 and the specific surface areathereof was 1.5 m²/g.

Comparative Example 11

Cylindrical non-aqueous electrolyte secondary battery was made up in amanner similar to the embodiment 21 except that no magnetic field isapplied to the discharge hole. The aspect ratio of the sample powderthus obtained was expressed as A=64 and the specific surface area was2.0 m²/g.

Test cells similar to the cases of the embodiments 5 to 9 were made upwith respect to carbon fibers used in the respective embodiments and thecomparative examples to measure charge/discharge ability (capacity). Inaddition, in the test cell, value obtained by subtracting dischargecapacity from charge capacity was calculated as capacity loss. Themeasured result is shown in Table 6.

TABLE 6 FIBER SPECIFIC CAPACITY CROSS FIBER FIBER ASPECT SURFACECAPACITY MAINTE- SECTIONAL DIAMETER LENGTH RATIO AREA CAPACITY LOSSNANCE STRUCTURE L/μm T/μm A m²/g mAh/g mAh/g RATIO % EMBODIMENT RANDOM-19 25 1.3 0.9 280 20 93 21 RADIAL EMBODIMENT RANDOM- 19 63 3.3 0.8 28015 92 22 RADIAL EMBODIMENT RANDOM- 21 148 7.0 1.2 280 25 88 23 RADIALEMBODIMENT RANDOM- 20 186 9.3 1.3 280 25 85 24 RADIAL EMBODIMENT RANDOM-23 950 41.0 1.5 280 30 80 25 RADIAL COMPARATIVE RANDOM- 20 1280 64.0 2.0280 50 70 EXAMPLE 11 RADIAL

Further, with respect to cylindrical batteries made up in the respectiveembodiments and the comparative examples, there was repeatedly carriedout charge/discharge cycle in which 2.5 hour constant current/constantvoltage charge operation is conducted under the condition of chargecurrent of 1 A and the maximum charge voltage of 4.2 V, and dischargeoperation is then conducted under the condition of discharge current 700mA until voltage falls down to 2.75 V to determine ratio of capacity ofthe 100-th cycle with respect to capacity of the second cycle (capacitymaintenance ratio). The result of the capacity maintenance ratio of the200-th cycle with respect to the second cycle is shown in theabove-mentioned Table 6. In addition, the relationship between theaspect ratio A and the capacity maintenance ratio is shown in FIG. 24.

From the above-described result, it has been found that carbon fiberhaving cross sectional portions different in the crystal structureperiodically in the fiber length direction is crushed, thereby making itpossible to easily realize low aspect ratio, and that in the case wheresuch crushed powder is used as anode material, non-aqueous electrolytesecondary battery excellent in the cycle characteristic can be obtained.

What is claimed is:
 1. An anode material for a non-aqueous electrolytesecondary battery comprising carbon fiber capable of carrying outdoping/undoping of lithium, the carbon fiber having an areareplenishment rate of at least 0.8, the area replenishment rate beingdefined by dividing an area of a cross section of the carbon fiber by aminimum area of a circumscribed rectangle surrounding the cross section,the carbon fiber further being comprised of graphitized carbon fiber,the graphitized carbon fiber having a true density of at least 2.1g/cm³.
 2. The anode material for the non-aqueous electrolyte secondarybattery as set forth in claim
 1. wherein a circularity is defined bydividing a circumferential length of a circle by a contour length of thecross section of carbon fiber, the circle having a same area as that ofthe cross section of the carbon fiber, the circularity being at least0.8 and less than 1.0.
 3. The anode material for the non-aqueouselectrolyte secondary battery as set forth in claim 1, wherein a bulkdensity of the graphitized carbon fiber is at least 0.4 g/cm³.
 4. Theanode material for the non-aqueous electrolyte secondary battery as setforth in claim 1, wherein a specific surface area of the graphitizedcarbon fiber is up to and including 9 m²/g.
 5. The anode material forthe non-aqueous electrolyte secondary battery as set forth in claim 1,wherein the graphitized carbon fiber has a grain size distributionwherein an accumulated 10% particle diameter is at least 3 μm, anaccumulated 50% particle diameter is at least 10 μm, and an accumulated90% particle diameter is up to and including 70 μm.
 6. The anodematerial for the non-aqueous electrolyte secondary battery as set forthin claim 1, wherein the carbon fiber has a thickness T of the thinnestportion of carbon fiber, an axis length L in a long axis and length W ina direction perpendicular to the long axis, the carbon fiber furtherhaving a shape parameter X up to and including 125 calculated by thefollowing formula: X=(W/T)×(L/T).
 7. A non-aqueous electrolyte secondarybattery comprising: an anode comprising carbon material, the carbonmaterial being capable of carrying out doping/undoping of lithium, thebattery further comprising a cathode, and a non-aqueous electrolyticsolution in which an electrolyte is dissolved in a non-aqueous solvent,the carbon material comprising carbon fiber, the carbon fiber having anarea replenishment rate of at least 0.8, the area replenishment ratedefined by dividing an area of a cross section of the carbon fiber by aminimum area of a circumscribed rectangle surrounding the cross section,the carbon fiber further being comprised of graphitized carbon fiber,the graphitized carbon fiber having a true density of at least 2.1g/cm³.